1. Field of the Invention
The present invention relates to an image forming apparatus in which a length of a sheet conveying pathway extending from a feeding device to a recording unit is shorter than a conveyance direction length of a maximum usable size sheet, and a control method therefor.
2. Description of the Related Art
There is known a small-sized image forming apparatus in which a sheet conveyance distance from a sheet feeding unit to an image recording unit is relatively short. When conveying a sheet to the image recording unit in synchronization with image recording timing, therefore, it is necessary to drive the feeding device and assist the sheet conveyance to adjust the image position.
If the sheet size set by a user is different from the size of the sheet actually set in an image forming apparatus, such as a printer, and if the length in the sheet conveyance direction (hereinafter, referred to as the “conveyance direction length”) of the actually set sheet is shorter than the conveyance direction length of the user-set sheet size, a second sheet is also fed and conveyed from the sheet feeding unit and is consequently wasted.
Further, in the case of an image forming apparatus that heats and fixes a toner image on a sheet by using a fixing roller or a fixing belt, the image is formed on the leading end of the second sheet, and thus the second sheet winds around the fixing roller or the fixing belt.
Methods addressing the above-described issues have been proposed which include detecting the sheet size in accordance with the set position of a side fence or an end fence that regulates the position of a sheet set in a sheet storage unit of the image forming apparatus, and detecting the length of a sheet fed and conveyed from the sheet feeding unit by providing the sheet storage unit with a device for detecting the trailing end of the sheet.
Further, to prevent the second sheet from winding around the fixing roller when the user-set sheet size is different from the size of the sheet actually set in the sheet feeding unit, a method has been proposed which temporarily stops a sheet feeding roller in a sheet feeding operation immediately after the sheet feeding roller has fed a sheet by a distance corresponding to half the conveyance direction length of the user-set sheet size, rotates the sheet feeding roller again, determines, if a sheet detection sensor detects a sheetless state during the sheet conveying process, that the actually set sheet size is different from the user-set sheet size, and stops the sheet conveyance. According to this method, to prevent the sheet feeding roller from acting as a load during the temporary stoppage thereof when the user-set sheet size matches the size of the sheet actually set in the sheet feeding unit, gate rollers (i.e., a pair of registration rollers) are provided between the sheet feeding roller and a transfer unit to correct oblique conveyance of the sheet by abutting the sheet against the gate rollers and flexing the sheet. The amount of flexure of the sheet is set to be larger than the amount of conveyance of the sheet conveyed by the gate rollers during the stop time in which the sheet feeding roller is temporarily stopped.
Further, according to another method, to detect the length of the fed and conveyed sheet in an early stage a sheet feeding device is provided with a trailing end detection device that detects that the trailing end of the fed sheet has passed a predetermined trailing end detection position in a manual feed tray.
According to the above-described method of detecting the size of the sheet set in the sheet storage unit, however, it is necessary to install a size detection mechanism and a sheet trailing end detection device, which operate in conjunction with the side fence or the end fence. Therefore, the image forming apparatus is increased in size and cost. Further, in manual sheet feeding using a sheet storage unit exposed to the outside of the image forming apparatus, the regulation of the sheet set position is limited to the regulation in the sheet width direction by the side fence. Thus, the detection of the conveyance direction size of the sheet by the use of the end fence is not performed, and there is difficulty in providing the sheet trailing end detection device.
Further, according to the first method described above, if the conveyance direction length of the actually set sheet is equal to or less than half the conveyance direction length of the user-set sheet size, a gap is formed between the first and second sheets and allows the sheet detection sensor to detect the sheetless state. However, if the conveyance direction length of the actually set sheet is more than half the conveyance direction length of the user-set sheet size, e.g., if the user-set sheet size is A4 portrait and the actually set sheet size is A5 portrait, no gap is formed between the first and second sheets. Thus, the sheet detection sensor fails to detect the sheetless state, and the sheet conveyance is not stopped. As a result, the second sheet is wasted.
Further, according to the first method described above, a gap of approximately 15 mm to approximately 20 mm is required for the detection of the sheetless state by the sheet detection sensor using a swing lever (feeler), which is normally used in a small-sized printer. However, the amount of flexure of the sheet for correcting the oblique conveyance of the sheet is normally approximately 2 mm to approximately 4 mm. If the amount of flexure of the sheet is increased, therefore, the sheet interferes with a conveying guide, and results in an increase in conveyance noise and damage to the sheet such as crumpling. Further, in the case of a relatively thick sheet, in which an image failure such as shock jitter tends to occur, the resilience of the sheet is relatively high. Even if the amount of flexure of the sheet is set to a relatively large value, therefore, the sheet skids on the sheet feeding roller, and the actual amount of flexure of the sheet is not increased, resulting in image failure such as shock jitter.